Conventional resonant cavity filters consist of an outer housing made of an electrically conductive material and one or more resonant elements, or resonators, are mounted inside the housing. The resonators may be mounted within the cavity using, for example, a dielectric material. Electromagnetic energy is coupled through a first coupling mechanism in the housing to a first resonator and then to any additional resonators in the housing. A second coupling mechanism is used to output the electromagnetic energy from the housing.
Resonators are often used in filters to pass or reject certain signal frequencies. The particular design, shape, materials and spacing of the housing, the resonant elements, and the apertures between resonant elements determine the signal frequencies passed through the filter, as well as the insertion loss of the filter and quality factor (“Q”) of each resonator. Ideally, resonators should have minimum signal loss in their passbands.
Resonators generally consist of conductive structures, and are typically of either a two-dimensional type, or a three-dimensional type. Two-dimensional resonators, also known as microstrip resonators, are formed by depositing a conductive layer onto a substrate and removing some of the conductive material from the substrate to leave a length of conductive material behind. The length of conductive material remaining on the substrate forms one or more resonators. Two-dimensional resonators are commonly referred to as thin film resonators.
Thin film resonator technology has been used to produce high performance military and commercial wireless devices. One type of two-dimensional resonators uses a thin film of high temperature superconductive (HTS) material disposed onto a dielectric substrate. One major problem associated with the fabrication of thin film resonators is the variation in the thickness of the dielectric substrate. Thickness of the dielectric substrate influences not only the coupling coefficient between adjacent resonators, but also affects the resonant frequency of the resonator. Accordingly, variations in the thickness of the dielectric substrate also results in the variations in the resonant frequency of the thin film resonator.
The velocity of an electromagnetic wave in a microstrip is given by Equation 1.                               v          p                =                  c                                    ɛ              e                                                          Equation        ⁢                                   ⁢        1            Where c is the velocity of light in free space and εe is the effective dielectric constant of the microstrip. The effective dielectric constant of the microstrip can be approximated by Equation 2.                               ɛ          e                ≈                                            1              +                              ɛ                r                                      2                    +                                                    [                                                                            ɛ                      r                                        -                    1                                    2                                ]                            ⁡                              [                                  1                  +                                      10                    ⁢                                          h                      w                                                                      ]                                                    -                              1                2                                                                        Equation        ⁢                                   ⁢        2            Where ∈r is the dielectric constant of the substrate, h is the thickness of the substrate, and w is the width of the microstrip. As can be seen from Equations 1 and 2, when h increases, ∈e decreases and, therefore, υp increases. As a result, the resonant frequency of the microstrip resonator increases as well. In practice, it is not uncommon for even the most precisely fabricated substrates to vary in thickness by as much as ±1%.
Due to such dependence of the resonant frequency on the thickness of the substrate, the measured frequency response of such a microstrip resonator usually deviates from the frequency response for which the resonator is designed. Tuning of filters designed using such resonators is a very tedious task even for experienced filter engineers, because one has to tune not only the coupling coefficient between the resonators but also the resonant frequency of the individual resonators.
Another issue pertinent to thin film filters is the miniaturization of the resonator structure used to design such filters. As the resonant frequency of a microstrip resonator decreases, and, therefore, the resonant wavelength increases, it is necessary to use larger size microstrip resonators, which necessitates the use of bulky resonators to achieve lower resonant frequencies. Substantial effort has been devoted to the miniaturization of the resonator structures. FIG. 1 shows some exemplary thin film resonator structures that have been used in filters. In FIG. 1, reference numeral 12 refers to a standard microstrip resonator, reference numeral 14 refers to a loop resonator formed by removing the central portion from the standard microstrip resonator 12 and reference numeral 16 refers to a capacitively loaded loop resonator. Further, reference numeral 18 refers to an open loop resonator, reference numeral 20 refers to a meander shaped open loop resonator, and reference numeral 22 refers to a folded open loop resonator.